Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Glutamate produces its effects on central neurons by binding to and thereby activating cell surface receptors. These receptors have been divided into two major classes, the ionotropic and metabotropic glutamate receptors, based on the structural features of the receptor proteins, the means by which the receptors transduce signals into the cell, and pharmacological profiles.
The metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors that activate a variety of intracellular second messenger systems following the binding of glutamate. Activation of mGluRs in intact mammalian neurons elicits one or more of the following responses: activation of phospholipase C; increases in phosphoinositide (PI) hydrolysis; intracellular calcium release; activation of phospholipase D; activation or inhibition of adenyl cyclase; increases or decreases in the formation of cyclic adenosine monophosphate (cAMP); activation of guanylyl cyclase; increases in the formation of cyclic guanosine monophosphate (cGMP); activation of phospholipase A2; increases in arachidonic acid release; and increases or decreases in the activity of voltage-and ligand-gated ion channels. Schoepp et al., Trends Pharmacol. Sci. 14:13 (1993), Schoepp, Neurochem. Int. 24:439 (1994), Pin et al., Neuropharmacology 34:1 (1995), Bordi and Ugolini, Prog. Neurobiol. 59:55 (1999).
Molecular cloning has identified eight distinct mGluR subtypes, termed mGluR1 through mGluR8. Nakanishi, Neuron 13:1031 (1994), Pin et al., Neuropharmacology 34:1 (1995), Knopfel et al., J. Med. Chem. 38:1417 (1995). Further receptor diversity occurs via expression of alternatively spliced forms of certain mGluR subtypes. Pin et al., PNAS 89:10331 (1992), Minakami et al., BBRC 199:1136 (1994), Joly et al., J. Neurosci. 15:3970 (1995).
Metabotropic glutamate receptor subtypes may be subdivided into three groups, Group I , Group II, and Group III mGluRs, based on amino acid sequence homology, the second messenger systems utilized by the receptors, and by their pharmacological characteristics. Group I mGluR comprises mGluR1, mGluR5 and their alternatively spliced variants. The binding of agonists to these receptors results in the activation of phospholipase C and the subsequent mobilization of intracellular calcium.
Recent advances in the elucidation of the neurophysiological roles of mGluRs have established these receptors as promising drug targets in the therapy of acute and chronic neurological and psychiatric disorders and chronic and acute pain disorders. Because of the physiological and pathophysiological significance of the mGluRs, there is a need for new drugs and compounds that can modulate mGluR function.
Neurological, Psychiatric and Pain Disorders.
Attempts at elucidating the physiological roles of Group I mGluRs suggest that activation of these receptors elicits neuronal excitation. Various studies have demonstrated that Group I mGluRs agonists can produce postsynaptic excitation upon application to neurons in the hippocampus, cerebral cortex, cerebellum, and thalamus, as well as other CNS regions. Evidence indicates that this excitation is due to direct activation of postsynaptic mGluRs, but it also has been suggested that activation of presynaptic mGluRs occurs, resulting in increased neurotransmitter release. Baskys, Trends Pharmacol. Sci. 15:92 (1992), Schoepp, Neurochem. Int. 24:439 (1994), Pin et al., Neuropharmacology 34:1 (1995), Watkins et al., Trends Pharmacol. Sci. 15:33 (1994).
Metabotropic glutamate receptors have been implicated in a number of normal processes in the mammalian CNS. Activation of mGluRs has been shown to be required for induction of hippocampal long-term potentiation and cerebellar long-term depression. Bashir et al., Nature 363:347 (1993), Bortolotto et al., Nature 368:740 (1994), Aiba et al., Cell 79:365 (1994), Aiba et al., Cell 79:377 (1994). A role for mGluR activation in nociception and analgesia also has been demonstrated, Meller et al., Neuroreport 4: 879 (1993), Bordi and Ugolini, Brain Res. 871:223 (1999). In addition, mGluR activation has been suggested to play a modulatory role in a variety of other normal processes including synaptic transmission, neuronal development, apoptotic neuronal death, synaptic plasticity, spatial learning, olfactory memory, central control of cardiac activity, waking, motor control and control of the vestibulo-ocular reflex. Nakanishi, Neuron 13: 1031 (1994), Pin et al., Neuropharmacology 34: 1, Knopfel et al., J. Med. Chem. 38:1417 (1995).
Further, Group I metabotropic glutamate receptors and mGluR5 in particular, have been suggested to play roles in a variety of pathophysiological processes and disorders affecting the CNS. These include stroke, head trauma, anoxic and ischemic injuries, hypoglycemia, epilepsy, neurodegenerative disorders such as Alzheimer's disease and pain. Schoepp et al., Trends Pharmacol. Sci. 14:13 (1993), Cunningham et al., Life Sci. 54:135 (1994), Hollman et al., Ann. Rev. Neurosci. 17:31 (1994), Pin et al., Neuropharmacology 34:1 (1995), Knopfel et al., J. Med. Chem. 38:1417 (1995), Spooren et al., Trends Pharmacol. Sci. 22:331 (2001), Gasparini et al. Curr. Opin. Pharmacol. 2:43 (2002), Neugebauer Pain 98:1 (2002). Much of the pathology in these conditions is thought to be due to excessive glutamate-induced excitation of CNS neurons. Because Group I mGluRs appear to increase glutamate-mediated neuronal excitation via postsynaptic mechanisms and enhanced presynaptic glutamate release, their activation probably contributes to the pathology. Accordingly, selective antagonists of Group I mGluR receptors could be therapeutically beneficial, specifically as neuroprotective agents, analgesics or anticonvulsants.
Further, it has also been shown that mGluR5 antagonists are useful for the treatment of addictions or cravings (for drugs, tobacco, alcohol, any appetizing macronutrients or non-essential food items).
Recent advances in the elucidation of the neurophysiological roles of metabotropic glutamate receptors generally and Group I in particular, have established these receptors as promising drug targets in the therapy of acute and chronic neurological and psychiatric disorders and chronic and acute pain disorders.
Medical Use
The group I receptor, mGluR5, has been implicated in a number of central nervous system disease states, including pain (Salt and Binns, 2000; Bhave, et al., 2001), anxiety (Spooren, et al., 2000; Tatarczynska, et al., 2001), addiction to cocaine (Chiamulera, et al., 2001) and schizophrenia (Chavez-Noriega, et al., 2002). The N-methyl-D-aspartate (NMDA) receptor, an ionotropic glutamate receptor, has also been implicated in physiological and pathological processes. Of specific interest, blockade of NMDA receptors produces a transient state of psychosis and schizophrenia-like cognitive deficits (Krystal, et al., Arch Gen Psychiatry, 51: 199-214, 1994; Lahti, et al., Neuropsychopharmacol., 13: 9-19, 1995; Newcomer, et al., Neuropsychopharmacol., 20:106-118, 1999). Pharmacological manipulation of NMDA receptor function may be critical for the treatment of many neurological and psychiatric disorders such as epilepsy, Alzheimer's disease, drug dependence and schizophrenia (Kemp and McKernan, 2002). A functional interaction between NMDA receptors and mGluR5 has been demonstrated at a cellular level and at a behavioral level. Thus, activation of Group I mGluRs by DHPG enhanced NMDA-receptor mediated responses in mouse CA1 pyramidal neurones (Mannaioni, et al., J. Neurosci., 21:5925-5934, 2001). This effect was inhibited by MPEP, demonstrating that NMDA receptor function was enhanced through activation of mGluR5 (Mannaioni, et al., J. Neurosci., 21:5925-5934, 2001). Modulation of mGluR5 also altered the cognitive and behavioral abnormalities associated with NMDA receptor deficiency (Homayoun, et al., Neuropsychopharmacol., 29: 1259-1269, 2004). Together these data suggest that potentiation of mGluR5 could be beneficial in the treatment of disorders such as schizophrenia.
Non-Medical Use
In addition to their use in therapeutic medicine, the compounds of Formula I, as well as salts and hydrates of such compounds, are useful as pharmacological tools in the development and standardization of in vitro and in vivo test systems for the evaluation of the effects of potentiators of mGluR related activity in laboratory animals such as cats, dogs, rabbits, monkeys, rats and mice, as part of the search for new therapeutic agents.